1. Field of the Invention
The present invention relates to a recording ink composition suitable for an ink-jet printer, aqueous writing utensils, various kinds of recorders and pen plotters, and more particularly to a recording ink composition capable of producing color images on a sheet of plain paper by the ink-jet printing method. In addition, the present invention relates to an ink-jet printing method using the above-mentioned recording ink composition.
2. Discussion of Background
Ink-jet printers have been widely utilized in recent years because of the advantages of low noise and low running cost, and color ink-jet printers capable of producing color images on a sheet of plain paper have also been placed on the market.
With respect to such ink-jet printers, however, it is required to satisfy all the requirements, such as excellent color reproduction of color image, high water- and light-resistances of the obtained image, sufficient drying characteristics of the image, high image quality without blurring, and the reliability of ink-ejection from the printer.
Dyes and pigments for use in the ink composition have been improved in order to satisfy both the requirements of the color reproduction of color images formed on a sheet of plain paper and the light resistance and water resistance of such color images at the same time. However, it was difficult to obtain an ink composition capable of producing color images with the improved light and water resistances without the decrease of reliability of ink-ejection and the deterioration of color reproduction.
For instance, there are disclosed cyan color ink compositions in Japanese Laid-Open Patent Applications 63-210175, 1-126381 and 1-19077, each of which comprises a carboxylic-group-introduced copper phthalocyanine dye. Although the water resistance of the image obtained by use of the above-mentioned ink composition is superior, the light resistance thereof is inferior, as compared with the water and light resistances of the image obtained by use of an ink composition comprising a sulfonic-group-introduced phthalocyanine dye. Due to such insufficient light resistance of the recorded image, the color image tends to fade by the application of light thereto, so that the above-mentioned ink composition is not suitable for printing a poster which is bound to be exposed to light for an extended period of time.
It is conventionally proposed to use various pigments for the ink composition in order to obtain sufficient water resistance and light resistance of the recorded ink image. However, since the color tone of the pigments is unsatisfactory, such an ink composition as comprises a pigment is now only applied to a large-sized plotter which is required to produce images with extremely high light resistance. In addition, when the aqueous ink composition comprising a pigment is set in an ink-jet printer which is designed to produce images with high resolution, the nozzle of the printer easily tends to be clogged with the ink composition, so that the reliability of the ink ejection becomes very low.
Therefore, it is proposed to use a pigment and a dye in combination for the ink composition for improving the color tone of the ink composition so as to upgrade the color reproduction of the obtained color images, improving the light resistance and water resistance of the obtained image, and preventing the reliability of the ink ejection performance from decreasing. For example, Japanese Patent Publication 60-45668 discloses a recording ink composition which comprises water, a water-soluble blue dye and blue pigment, and a polymeric dispersant. However, not only the light resistance and water resistance of the dye component are insufficient, but also the penetration performance of the obtained ink composition through a sheet of plain paper and the color development of color image are not always satisfactory.
When the color images are produced, in particular, by using the color ink-jet printer, image blurring easily takes place in a secondary color image portion of a red, green or blue color obtained by superimposing the two colored inks and the boundaries thereof, even though the image clearness is sufficient in an image portion of a yellow, magenta or cyan color prepared by use of the ink composition of a single color.
Particularly, when the ink deposited on a sheet of paper is dried without any specific image-fixing unit, the drying characteristics of the ink are improved by increasing the penetrating property of the ink through the paper, as described in Japanese Laid-Open Patent Application 55-29546. However, because of the increase of the penetrating property of the ink through paper, obtained images are easily blurred and the sharpness of the obtained images is decreased depending on the kind of recording paper to be employed.
It is described in Japanese Patent Publication 60-23793 that the drying characteristics of the images obtained by ink-jet recording can be improved without decrease of the sharpness of the obtained image when a dialkylsulfosuccinate is contained as the surfactant in the ink composition. However, when such an ink composition is used for ink-jet printing, the diameter of a picture element of the obtained image conspicuously varies depending on the kind of image receiving sheet to be employed. As a result, the image density of the printed images is considerably decreased, and the sharpness of the printed images is lowered. In addition, this kind of surfactant is easily decomposed if an ink composition is alkaline, so that the activating effect of the surfactant is impaired during the storage of the ink composition. The result is that the wettability of the recording paper by this ink composition is lowered, so that the drying characteristics of the recorded image are changed. Thus, the image deterioration will occur at the boundaries of the secondary color image.
An ink composition comprising a strongly basic material is disclosed in Japanese Laid-Open Patent Application 56-57862 for the purpose of improving the drying characteristics of the images recorded by ink-jet printing. The drying characteristics of the image formed on a so-called acidic paper which has been sized by use of rosin can be improved. However, the improvement in drying characteristics is not observed when the image is printed on a paper which is prepared by use of a sizing agent such as alkyl ketene dimer or alkenyl sulfosuccinic acid. Even when the acidic paper is employed, the drying characteristics of the secondary color image portion formed by superimposing the two colored inks are still insufficient.
There is proposed a recording ink composition comprising a polyhydric alcohol derivative and pectin as disclosed in Japanese Laid-Open Patent Application 1-203483. In this ink composition, pectin is contained as a thickening agent to prevent the blurring of ink image obtained on the paper. However, since pectin is a nonionic material having a hydroxyl group as a hydrophilic group, the reliability of ink ejection is decreased, for example, when the printing operation is resumed after intermission.